1. Field of the Invention
The present invention relates to an imaging device for capturing an image with applying illuminating light emitted from a light source to a part to be observed through a light exit window, and a method of cleaning the light exit window of the imaging device.
2. Description of the Related Art
Conventionally, endoscopic systems for observing a tissue in a body cavity are widely known, and electronic endoscopic systems which acquire a normal image by imaging a part to be observed in a body cavity with applying white light and display the normal image on a monitor screen have been put to practical use and are widely used.
As one of such endoscopic systems, an endoscopic system for acquiring a fluorescence image of blood vessels has been proposed, where ICG (indocyanine green) is administered into the body in advance and near-infrared light is applied to a part to be observed to detect fluorescence emitted from the ICG in the blood vessels, in order to check blood vessels that are present at a depth exceeding a certain depth from the surface of a living tissue of the subject and thus do not appear on the normal image.
When an endoscopic surgery is performed with using the above-described type of endoscopic system, smoke or mist adheres to the distal end of the insertion section of the endoscope inserted into the body cavity and may often contaminate the imaging lens and the light exit window of the endoscope. At each such time, the endoscope is pulled out from the body cavity and cleaned.
In particular, the above-described type of endoscopic system, which observes fluorescence in the near-infrared region, usually uses diffused coherent laser light, in order to apply near-infrared laser light to a wide range and to avoid concentration of the near-infrared laser light on the surface of the part to be observed exposed to the near-infrared laser light, and it is generally necessary to provide a light exit window with a large area for the near-infrared light.
Therefore, there is high possibility of the above-described adhesion of contaminants to the light exit window for the near-infrared light, and frequent cleaning of the endoscope is required. However, if the endoscope is pulled out from the body cavity each time contaminants adhere to the light exit window and is again inserted into the body cavity after cleaning, the surgery operation stops during the cleaning and the field of view of the reinserted endoscope is changed from that before, causing significant loss for the time of the surgery operation.
Further, in the case where a light source with a relatively high power density, such as a laser light source, is used in the above-described type of endoscopic system, when contaminants, such as blood, adhere to the light exit window provided at the distal end of the endoscope, the contaminants may be burned by the illuminating light and the burnt contaminants on the light exit window hinder application of necessary illumination. In addition, the burnt contaminants on the light exit window and increase of reflected light from the light exit window may increase the temperature of a light guiding member extending to the light exit window and the light guiding member may be burned.
In order to address such problems, Japanese Unexamined Patent Publication Nos. 2009-279291, 2008-279202, H09-253034 and H07-289514 (hereinafter, Patent Documents 1 to 4), for example, propose endoscopic systems provided with a cleaning mechanism at the distal end of the insertion section of the endoscope, so that the imaging lens and the light exit window can be cleaned without pulling out the insertion section of the endoscope from the body cavity.
With the endoscopic systems disclosed in Patent Documents 1 to 4, when contaminants adhere to the surface of the imaging lens disposed at the distal end of the insertion section during observation of the normal image, for example, the presence of contaminants adhering to the distal end of the insertion section can be checked by the operator by observing the actually captured image. However, when the contaminants adhere only to the light exit window for the near-infrared light during observation of the normal image, for example, the operator does not notice the presence of adhering contaminants. Further, when the observation of the normal image is switched to observation of the fluorescence image, it is also difficult to notice decrease of illuminance since the near-infrared light is invisible, and the above-described problem of burnt contaminants on the light exit window may occur.